Oil and Natural Gas Operations (Unaudited) (Oil and Gas Operations) (Details)	12 Months Ended
Dec. 31, 2014 MMBoe	Dec. 31, 2013 MMBoe	Dec. 31, 2012 MMBoe
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period, total	347.8	346.4	343.1
Revisions of previous estimates, total	(75.7)	4.6	(42.1)
Purchases, total	0.1	0.2	12.4
Extensions and discoveries, total	130.0	36.8	57.1
Production, total	(25.8)	(25.4)	(24.1)
Sales, total	(3.7)	(14.8)
Proved reserves at end of period, total	372.7	347.8	346.4
Proved developed reserves at end of period (BOE)	264.5us-gaap_ProvedDevelopedReservesBOE1	259.8us-gaap_ProvedDevelopedReservesBOE1	260.5us-gaap_ProvedDevelopedReservesBOE1
Proved undeveloped reserves at end of period (BOE)	108.2us-gaap_ProvedUndevelopedReserveBOE1	88.0us-gaap_ProvedUndevelopedReserveBOE1	85.9us-gaap_ProvedUndevelopedReserveBOE1
Oil Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	164,870	155,348	129,578
Revisions of previous estimates	(48,548)	(680)	(8,546)
Purchases	88	142	7,950
Extensions and discoveries	76,722	20,517	35,132
Production	(11,818)	(10,378)	(8,766)
Sales	(87)	(79)
Proved reserves at end of period	181,227	164,870	155,348
Proved developed reserves at end of period (volume)	118,697	113,795	105,976
Proved undeveloped reserves at end of period (volume)	62,530	51,075	49,372
Natural Gas Liquids Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	63,011	56,155	53,957
Revisions of previous estimates	(15,165)	2,211	(9,557)
Purchases	26	56	2,569
Extensions and discoveries	29,695	7,823	11,759
Production	(4,104)	(3,233)	(2,573)
Sales	0	(1)
Proved reserves at end of period	73,463	63,011	56,155
Proved developed reserves at end of period (volume)	47,621	42,087	36,440
Proved undeveloped reserves at end of period (volume)	25,842	20,924	19,715
Natural Gas Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	719,725	809,128	957,368
Revisions of previous estimates	(71,806)	18,465	(143,704)
Purchases	116	282	10,656
Extensions and discoveries	141,209	50,568	61,170
Production	(59,562)	(70,506)	(76,362)
Sales	(21,756)	(88,212)
Proved reserves at end of period	707,926	719,725	809,128
Proved developed reserves at end of period (volume)	589,074	623,305	708,657
Proved undeveloped reserves at end of period (volume)	118,852	96,420	100,471

Proved Developed And Undeveloped Reserves, Net, Energy

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Proved Developed And Undeveloped Reserves, Revisions Of Previous Estimates, Increase Decrease, Equivalent

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Proved Developed And Undeveloped Reserves, Sales Of Minerals In Place, Equivalent

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Proved Development And Undeveloped Reserves, Extensions, Discoveries, And Additions, Equivalent

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Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

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The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

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Production of proved reserves.

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Purchase of minerals in place.

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Revisions represent changes in previous estimates of proved reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors.

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Sales of minerals in place.

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Energy measure of interests in proved developed reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas.

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Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

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Energy measure of proved undeveloped reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas.

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Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

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